During a trip to the hardware store, Yui Hirasawa finds Azusa Nakano gazing at a pig-nosed, soft-shelled turtle in a tank. Yui assumes that Azusa likes it (although Azusa just thought it looked odd) and convinces the rest of the club to buy it for her as a "kōhai" to take care of after they received some money by selling Sawako Yamanaka's old guitar. Despite being indifferent towards the turtle at first, Azusa became attached to it after some time. Yui gave the turtle the name Ton, and clearly adores it more than anyone else.

In the episode Hot!, Ton started to molt its skin, leading the club to get a new tank for it. During the summer break, Azusa took the turtle home to take care of it.

Towards the end of their final semester, Mio Akiyama feeds Ton for the first and last time, lamenting that she has not done so more often before graduation.

In the movie, Ton had the mission to decide the destination for the graduation trip by touching a certain tea cup. By coincidence, it touched Mio's cup first, deciding London as the destination.

Ui Hirasawa wins a large stuffed turtle from the batting cages, and keeps it on her bed.

There are at least five small statues of turtles on the banister of the stairway leading up to the music room.

In a flashback, it is revealed that Yui had a fascination (or at least momentary fascination) with turtles as she was playing with one while singing a song when her class was going on a walk.

Much of the care and maintenance for Ton comes from either Mugi or Azusa. Mugi has provided general knowledge and a larger tank, while Azusa is responsible for feeding. Yui generally tries to contribute by coming up with ideas for things to add to Ton's tank, but most of these would either prove harmful or pointless for a turtle.